This invention relates to rectifying devices and more particularly to trench Schottky barrier rectifiers as well as methods of forming these devices.
Rectifiers exhibit relatively low resistance to current flow in a forward direction and a high resistance to current flow in a reverse direction. Trench Schottky barrier rectifiers are a type of rectifier that have found use as output rectifiers in switching-mode power supplies and in other high-speed power switching applications such as motor drives. These devices are capable of carrying large forward currents and supporting large reverse blocking voltages.
U.S. Pat. No. 5,365,102 to Mehrotra et al. and entitled xe2x80x9cSchottky Barrier Rectifier with MOS Trenchxe2x80x9d, the entire disclosure of which is hereby incorporated by reference, discloses trench Schottky barrier rectifiers which have a higher breakdown voltage than is theoretically attainable with an ideal abrupt parallel-plane P-N junction. A cross-sectional representation of one embodiment of the described rectifiers is illustrated in FIG. 1. In this figure, rectifier 10 includes a semiconductor substrate 12 of first conductivity type, typically N-type conductivity, having a first face 12a and a second opposing face 12b. The substrate 12 comprises a relatively highly doped cathode region 12c (shown as N+) adjacent the first face 12a. A drift region 12d of first conductivity type (shown as N) extends from the cathode region 12c to the second face 12b. Accordingly, the doping concentration of the cathode region 12c is greater than that of the drift region 12d. A mesa 14 having a cross-sectional width xe2x80x9cWmxe2x80x9d, defined by opposing sides 14a and 14b, is formed in the drift region 12d. The mesa can be of stripe, rectangular, cylindrical or other similar geometry. Insulating regions 16a and 16b (described as SiO2) are also provided on the mesa sides. The rectifier also includes an anode electrode 18 on the insulating regions 16a, 16b. The anode electrode 18 forms a Schottky rectifying contact with the mesa 14 at second face 12b. The height of the Schottky barrier formed at the anode electrode/mesa interface is dependent on the type of electrode metal and semiconductor (e.g., Si, Ge, GaAs, and SiC) used and is also dependent on the doping concentration in the mesa 14. Finally, a cathode electrode 20 is provided adjacent the cathode region 12c at the first face 12a. The cathode electrode 20 ohmically contacts cathode region 12c. 
In a process described in U.S. Pat. No. 5,365,102, drift region 12d is provided by epitaxial growth on substrate 12c. Trenches are then etched through photoresist-patterned nitride layers, forming discrete mesas 14 having thermal oxidation resistant nitride caps. Insulating regions 16, preferably silicon dioxide, are formed on the trench sidewalls and bottoms 22b, but not on the tops of the mesas 14 (faces 12b) because of the presence of the nitride regions. The nitride regions (as well as any stress relief oxide regions, if present) are removed, and anode 18 and cathode 20 metallization provided. For more information, see U.S. Pat. No. 5,365,102.
As is discussed more fully below, the present invention concerns improvements in trench Schottky barrier rectifiers related to those in U.S. Pat. No. 5,365,102 and to processes for making such trench Schottky barrier rectifiers.
According to an embodiment of the invention, a method of forming a trench Schottky barrier rectifier is provided. The method comprises:
(a) Forming a semiconductor region having first and second opposing faces. The semiconductor region comprises a drift region of first conductivity type adjacent the first face and a cathode region of the first conductivity type adjacent the second face. The drift region has a lower net doping concentration than the net doping concentration associated with the cathode region.
(b) Forming a plurality of trenches that extend into the semiconductor region from the first face. These trenches define a plurality of mesas within the semiconductor region and form trench intersections at a plurality of locations.
(c) Providing an oxide layer that covers the semiconductor region at locations that correspond to trench bottoms and lower portions of the trench sidewalls.
(d) Providing a polysilicon region that is disposed within the trenches over the oxide layer.
(e) Providing insulating regions over the oxide layer at the trench intersections.
(f) Providing an anode electrode that is adjacent to and forms a Schottky rectifying contact with the drift region.
Where desired, the rectifier can be provided with a cathode electrode on the second face of the semiconductor region.
The semiconductor is preferably a silicon semiconductor and has n-type conductivity. Preferred insulating regions are borophosphosilicate glass regions.
The step of forming the semiconductor region preferably includes providing a semiconductor substrate corresponding to the cathode region, and subsequently growing an epitaxial semiconductor layer corresponding to the drift region on the substrate.
The step of forming the trenches preferably comprises: forming a patterned masking layer over the first face of the semiconductor region and etching the trenches through the masking layer. In some embodiments, the trenches are etched into the drift regions, but not into the cathode region. In others, the trenches are etched sufficiently deeply such that they extend through the drift region and into the cathode region.
The steps of forming the oxide layer, the polysilicon region, and the insulating regions preferably further comprise the following: (a) forming an oxide layer on the first face of the semiconductor region and within the trenches, for example, by thermal growth or by oxide deposition processes; (b) forming a polysilicon layer over the oxide layer; (c) etching the polysilicon layer such that that portions of the oxide layer are exposed over the first face, and portions of the oxide layer are exposed over upper portions of the trench sidewalls; (d) forming an insulating layer over the oxide layer and the etched polysilicon layer; (e) forming a patterned etch resistant layer over the insulating layer at the trench intersections; and (f) etching the insulating layer and the oxide layer where not covered by the patterned etch resistant layer.
According to another embodiment of the invention, a trench Schottky barrier rectifier is provided. The rectifier comprises:
(a) A semiconductor region having first and second opposing faces. The semiconductor region comprises a drift region of first conductivity type adjacent the first face and a cathode region of the first conductivity type adjacent the second face. The drift region has a lower net doping concentration than that of the cathode region.
(b) A plurality of trenches extending into the semiconductor region from the first face. The trenches define a plurality of mesas within the semiconductor region, and the trenches form a plurality of trench intersections.
(c) An oxide layer covering the semiconductor region on bottoms of the trenches and on lower portions of sidewalls of the trenches.
(d) A polysilicon region disposed over the oxide layer within the trenches.
(e) Insulating regions at the trench intersections that cover a portion of the polysilicon region and a portion of the oxide layer.
(f) An anode electrode that is adjacent to and forms a Schottky rectifying contact with the drift region.
A number of trench intersection angles are possible. In one preferred case, the trenches intersect at right angles to one another. A number of configurations are possible for the insulating regions at the trench intersections. In one preferred instance, the insulating regions are rectangular when viewed from above the trenches.
One advantage of the present invention is that trench Schottky barrier rectifiers, in which cells are defined by intersecting trenches, can be formed in high yield, due to improved process control at trench intersection regions.
Another advantage of the present invention is that such trench Schottky barrier rectifiers can be formed without substantial risk of degradation in xe2x80x9cpinch offxe2x80x9d at the trench intersection regions. Degradation of this type serves to decrease reverse bias breakdown voltages and increase leakage currents.
These and other embodiments and advantages of the present invention will become readily apparent to those skilled in the art upon review of the disclosure to follow.